1. Field of the Invention
The present invention relates to a motor drive apparatus equipped with a dynamic braking control unit.
2. Description of the Related Art
In a motor drive apparatus for driving a synchronous motor used to drive a feed shaft of a machine tool or industrial machine or an industrial robot or the like, dynamic braking that produces a dynamic braking force by short-circuiting the motor windings of the synchronous motor is widely used in the case of an emergency stop or in an emergency such as occurrence of an alarm (warning). If electrically disconnected from the power supply, the synchronous motor which uses a permanent magnet retains a magnetic field flux, and the rotating motor acts as a generator; therefore, dynamic braking can be applied by short-circuiting the windings of the rotating motor.
In dynamic braking, from the standpoint of reducing the braking distance and braking time by generating as large a braking torque as possible, it is preferable to short-circuit the motor phase windings via a dynamic braking resistor having a resistance value that matches the characteristics of the synchronous motor so that the rotational energy of the synchronous motor can be quickly converted into Joule heat and dissipated through the dynamic braking resistor as well as the resistance of the motor windings. However, in order to short-circuit the motor windings via the dynamic braking resistor in an emergency, a switching unit for switching the connection must be provided separately from the semiconductor switching devices constituting the inverter provided for driving the synchronous motor.
FIG. 11 is a diagram showing the circuit configuration of a conventional motor drive apparatus that applies dynamic braking using mechanical contacts and resistors. FIG. 12 is an equivalent circuit diagram showing the motor drive apparatus of FIG. 11 when all the semiconductor switching devices in the inverter are turned off. FIG. 13 is a flowchart for explaining the operating principle of how dynamic braking is applied using resistors and mechanical contacts in the motor drive apparatus of FIG. 11.
As shown in FIG. 11, generally a synchronous motor 2 which uses a permanent magnet and which is driven by the motor drive apparatus 100 is supplied with AC drive power from the inverter 11 that converts the DC power supplied from the DC input side Vdc into the AC power. The inverter 11 is configured as a full-bridge inverter having an upper arm A and a lower arm B each provided, for example, with semiconductor switching devices (power transistors) and free-wheeling diode connected in reverse parallel with the respective semiconductor switching devices. A control unit 110 supplies to the inverter 11 a switching command for controlling the on/off operation of the semiconductor switching devices. With the semiconductor switching devices controlled on and off in accordance with the switching command, the inverter 11 converts the input DC to AC of a desired frequency for driving the synchronous motor 2. Though not specifically illustrated, generally a converter which converts AC supplied from a commercial AC power supply into DC for output is provided on the DC input side Vdc of the inverter 11.
Usually, a motor drive apparatus for driving a synchronous motor used to drive a feed shaft of a machine tool or industrial machine or an industrial robot or the like is equipped with a safety device for causing the motor drive apparatus to stop when an alarm occurs in order to protect the synchronous motor as well as the motor drive apparatus, for example, from a fault condition such as overcurrent or overload. The motor drive apparatus may also be provided with an emergency stop button which the user (operator) operates to cause the synchronous motor to stop when an emergency situation has occurred for some reason.
In the motor drive apparatus 100, when an emergency occurs such as an emergency stop or an alarm condition, the control unit 110 receives an alarm notification signal from the safety device or an emergency stop signal generated by operating the emergency stop button, whereupon the control unit 110 stops supplying the switching command for converting DC to AC to the semiconductor switching devices in the inverter 11, and instead supplies a switching command for applying dynamic braking to the synchronous motor 2 (hereinafter referred to as the “dynamic braking command”).
The operating principle of the dynamic braking using mechanical contacts will be described below with reference to the flowchart of FIG. 13. First, in step S201 of FIG. 13, the control unit 110 in the motor drive apparatus 100 receives the emergency stop signal or the alarm notification signal. Thereupon, the control unit 110 stops outputting the switching command for converting DC to AC, and instead outputs the switching command (dynamic braking command) for applying dynamic braking to the synchronous motor 2. More specifically, in step S202, the control unit 110 outputs a command for causing all the semiconductor switches in the inverter 11 to turn off, and then, in step S203, supplies to a short-circuiting unit 112 a command for short-circuiting the motor phase windings of the synchronous motor 2.
As shown in FIGS. 11 and 12, the short-circuiting unit 112 includes a selector switch having mechanical contacts, such as a relay or a magnetic contactor, and dynamic braking resistors Ru, Rv, and Rw connected to the selector switch. When the contacts of the selector switch in the short-circuiting unit 112 are closed in step S203, the motor phase windings of the synchronous motor 2 are short-circuited via the dynamic braking resistors Ru, Rv, and Rw. Even when the inverter 11 is disconnected from the DC power supply Vdc by turning off all the semiconductor switching devices in the inverter 11 in step S202 of FIG. 13, the synchronous motor 2 which uses a permanent magnet retains the magnetic field flux; as a result, after the DC power supply Vdc is disconnected, the synchronous motor 2 rotating by inertia acts as a generator, and electromotive forces Euv, Evw, and Euw are produced. At this time, the rotational energy of the synchronous motor 2 is quickly converted into Joule heat by the dynamic braking resistors Ru, Rv, and Rw connected across the motor phase windings of the synchronous motor 2 (step S203 of FIG. 13), thus generating a dynamic braking force.
Instead of the selector switch having mechanical contacts, such as a relay or a magnetic contactor, dynamic braking according to an alternative method may use a semiconductor switching device such as a power transistor (provided separately from the semiconductor switching devices constituting the inverter) as the short-circuiting unit.
Another alternative method of dynamic braking produces a dynamic braking force by short-circuiting the motor windings of the rotating synchronous motor by utilizing the switching operation of the semiconductor switching devices constituting the inverter. According to this alternative method, each function of the dynamic braking resistors and the switching unit for switching the connection to the dynamic braking resistors is implemented by ingeniously designing the switching pattern of the semiconductor switching devices constituting the inverter 11, not by providing mechanical contacts as separate hardware as described with reference to FIGS. 11 to 13. FIG. 14 is a diagram showing the circuit configuration of a conventional motor drive apparatus that applies dynamic braking by utilizing the switching operation of the semiconductor switching devices constituting the inverter. FIG. 15 is an equivalent circuit diagram showing the motor drive apparatus of FIG. 14 when all the semiconductor switching devices in the lower arm of the inverter are turned on. FIG. 16 is a flowchart for explaining the operating principle of how dynamic braking is applied utilizing the switching operation of the semiconductor switching devices in the motor drive apparatus.
According to this method, normally the semiconductor switching devices constituting the inverter 11 are controlled on and off so as to convert DC to AC but, in the event of an emergency situation, dynamic braking is applied by turning off all the semiconductor switching devices provided in either one of the upper and lower arms A and B of the full-bridge inverter 11, while turning on all the semiconductor switching devices provided in the other arm.
The operating principle of the above dynamic braking will be described in further detail below with reference to the flowchart of FIG. 16. First, in step S301 of FIG. 16, the control unit 110 in the motor drive apparatus 100 receives the emergency stop signal or the alarm notification signal. Thereupon, the control unit 110 stops outputting the switching command for converting DC to AC, and instead outputs the switching command (dynamic braking command) for applying dynamic braking to the synchronous motor 2. More specifically, in step S302, the control unit 110 turns off all the semiconductor switching devices provided in either one of the upper and lower arms A and B (in the illustrated example, the upper arm A) of the full-bridge inverter 11, and turns on all of the semiconductor switching devices provided in the other arm (in the illustrated example, the lower arm B). As a result, the synchronous motor 2 previously connected to the respective component elements of the inverter 11 as shown in FIG. 14 is now connected as shown in FIG. 15, thus short-circuiting the motor phase windings of the rotating synchronous motor 2. When all of the semiconductor switching devices provided in the one arm (in the illustrated example, the upper arm A) are turned off, and all of the semiconductor switching devices provided in the other arm (in the illustrated example, the upper arm B) are turned on, the synchronous motor 2 rotating by inertia acts as a generator in the presence of the magnetic field flux that the synchronous motor 2 using a permanent magnet retains, and electromotive forces Euv, Evw, and Ewu are produced. At this time, the motor phase windings of the rotating synchronous motor 2 are short-circuited by the turning on of the semiconductor switching devices provided in the lower arm B of the inverter 11 (step S302 of FIG. 16) and the action of their associated free-wheeling diodes. As a result, the current generated by the electromotive forces Euv, Evw, and Euw flows between the motor phase windings through the respective semiconductor switching devices in the lower arm of the inverter 11, and the rotational energy of the synchronous motor 2 is converted into Joule heat by the resistance of the motor windings, thus generating a dynamic braking force. The dynamic braking based on the switching operation of the semiconductor switching devices described with reference to FIGS. 14 to 16 is implemented by directly utilizing the circuit configuration of the DC-AC converting inverter 11; therefore, unlike the case of the dynamic braking using the mechanical contacts described with reference to FIGS. 11 to 13, the short-circuiting between the motor phase windings of the synchronous motor 2 cannot be accomplished by using resistors provided separately.
In the case of the method that applies dynamic braking by short-circuiting the motor windings of the rotating synchronous motor 2 by utilizing the switching operation of the semiconductor switching devices as described with reference to FIGS. 14 to 16, since the rotational energy of the synchronous motor 2 is converted into Joule heat by using only the resistance of the motor windings, the braking time increases compared with the case of the dynamic braking based on the mechanical contacts that uses not only the resistance of the motor windings but also the resistors.
In view of this, there is proposed, for example, as disclosed in Japanese Patent No. 3279102, a method for shortening the braking time in dynamic braking, wherein first the motor is decelerated by controlling the current flowing to the motor during dynamic braking to a constant current by controlling the on/off operation of the power transistors (semiconductor switching devices) provided in either one of the upper and lower arms of the inverter for driving the motor, and thereafter, the motor windings are short-circuited via dynamic braking resistors by closing contacts.
Generally, in the case of a selector switch having mechanical contacts, such as a relay or a magnetic contactor, the contact life in terms of the number of open/close operations is greatly dependent on the voltage applied between the contacts during the open/close operation. When switching the mechanical contacts from the open state to the closed state, a phenomenon called chattering (or bouncing), in which the two contact surfaces rapidly repeat opening and closing, occurs for a finite period of time, until the two contacts are finally brought into a stably contacting relationship, thus settling into the closed state. However, if closing the mechanical contacts is attempted while a high voltage is being applied between the contacts, a spark due to an arc discharge or glow discharge occurs between the contacts during the chattering.
In the case of the dynamic braking using the mechanical contacts described with reference to FIGS. 11 to 13, even when the inverter 11 is disconnected from the DC power supply Vdc by turning off all the semiconductor switching devices in the inverter 11 in step S202 of FIG. 13, the synchronous motor 2 which uses a permanent magnet retains the magnetic field flux; as a result, the synchronous motor 2 rotating by inertia acts as a generator, producing the electromotive forces Euv, Evw, and Euw. The electromotive forces Euv, Evw, and Euw continue to be applied between the respective mechanical contacts during the interval from the time that all the semiconductor switching devices in the inverter 11 are turned off in step S202 until the time that the motor phase windings of the synchronous motor 2 are short-circuited in step S203 by the short-circuiting unit 11. The electromotive forces Euv, Evw, and Euw are generally of the same magnitude as the voltage applied to the synchronous motor 2 when driving the synchronous motor 2, and in the case of the synchronous motor 2 for industrial use, the magnitude may reach a voltage level exceeding 400 V. If the mechanical contacts are closed in step S203 by the short-circuiting unit 11 while such a high voltage is being applied between the mechanical contacts, chattering such as described above occurs, and a spark due to an arc discharge or glow discharge is generated. Since such a spark may fuse the mechanical contacts to each other or may wear the mechanical contacts, there has been the problem that the contact life significantly degrades, increasing the maintenance cost due to frequent replacement of the contacts, etc. There has also been the problem that in order to avoid such fusing or wearing of the mechanical contacts, mechanical contact components having a large contact capacity have to be used, which not only increases the cost but also makes it difficult to reduce the size of the apparatus.
If semiconductor switching devices such as power transistors (provided separately from the semiconductor switching devices constituting the inverter) are used for the short-circuiting unit instead of the mechanical contact components such as relays or magnetic contactors, the service life problem associated with the mechanical contacts, such as described above, can be avoided. However, in this case, in addition to the dynamic braking semiconductor switches, an insulated primary power supply for operating the semiconductor switches and an insulated control circuit for on-off control of the semiconductor switches have to be provided in the short-circuiting unit; there has therefore been the problem that this method not only increases the cost but also makes it difficult to reduce the size of the apparatus, compared with the method that implements the dynamic braking using mechanical contact components such as relays or magnetic contactors.
On the other hand, the method of dynamic braking that produces a dynamic braking force by short-circuiting the motor windings of the rotating synchronous motor by utilizing the switching operation of the semiconductor switching devices constituting the inverter, as described with reference to FIGS. 14 to 16, is advantageous over the method of dynamic braking using the mechanical contacts, because there is no need to provide mechanical contacts for short-circuiting the motor windings of the synchronous motor nor is there a need to provide a circuit for driving the mechanical contacts; it also offers a further advantage in that the concern associated with the maintenance of the mechanical contacts is eliminated. However, since the rotational energy of the synchronous motor is converted into Joule heat by using only the resistance of the motor windings, there has been the problem that the braking time increases, compared with the case of the dynamic braking based on the mechanical contacts that also uses the dynamic braking resistors.
By contrast, according to the technique disclosed in Japanese Patent No. 3279102, first the motor is decelerated by controlling the current flowing to the motor during dynamic braking to a constant current by controlling the on/off operation of the power transistors (semiconductor switching devices) provided in either one of the upper and lower arms of the inverter for driving the motor, and thereafter, the motor windings are short-circuited via dynamic braking resistors by closing contacts. More specifically, in the first half of the process, the armature current is maintained constant by controlling the on/off operation of the power transistors, thereby controlling the synchronous motor to decelerate at a constant rate, and in the second half of the process, when the induced voltage of the synchronous motor decreases due to the on/off control of the power transistors, and the armature current becomes no longer constant, the on/off control of the power transistors is stopped, and the contacts of dynamic braking relays are closed to short-circuit the motor phase windings via the dynamic braking resistors.
However, according to the technique disclosed in Japanese Patent No. 3279102, when switching the process from the first half to the second half, if the contacts of the dynamic braking relay are closed while the power transistors are held in the OFF state, which means that the contacts are closed while the induced voltage is being applied, then a spark is generated, causing the contacts to be fused together or the mechanical contacts to wear, and hence the problem that the contact life significantly degrades. Furthermore, according to the technique disclosed in Japanese Patent No. 3279102, when applying dynamic braking, the power transistors are controlled on and off so as to maintain the armature current constant; this requires the provision of a dynamic braking control circuit including an armature current detector, a rectifier, an adder, a comparator, a delay, etc., and hence the problem that the circuit configuration becomes complex and the cost increases.